Assessment of foraminal decompression following discoplasty using a combination of ex vivo testing and numerical tools

Percutaneous Cement Discoplasty (PCD) is a minimally invasive surgical technique to treat degenerated intervertebral discs. When the disc is severely degenerated, the vacuum observed in place of the nucleus pulposus can be filled with bone cement to restore the disc height, open the foramen space, and relieve pain. This study aimed to evaluate the foramen geometry change due to PCD, in the loaded spine. Cadaveric spines (n = 25) were tested in flexion and extension while Digital Image Correlation (DIC) measured displacements and deformations. Tests were performed on simulated pre-operative condition (nucleotomy) and after PCD. Registering DIC images and the 3D specimen geometry from CT scans, a 3D model of the specimens aligned in the experimental pose was obtained for nucleotomy and PCD. Foramen space volume was geometrically measured for both conditions. The volume of cement injected was measured to explore correlation with the change of foramen space. PCD induced a significant overall foraminal decompression in both flexion (foramen space increased by 835 ± 1289 mm3, p = 0.001) and extension (1205 ± 1106 mm3, p < 0.001), confirming that the expected improvements of PCD show also during spine motion. Furthermore, in extension when the foramen is the most challenged, the impact of PCD on the foramen correlated with the injected cement volume.


S1.1 -Cadaveric specimens
For this study, Functional Spinal Units (FSU) were extracted from 15 Caucasian lumbar spines (9 males/6 females) aged 35 to 86 y.o. Only specimens presenting intact endplates on computed tomography (CT) scan images were selected. The selection did not consider the degree of disc degeneration. The specimens were cleaned of the soft tissue keeping intact the anterior longitudinal, posterior longitudinal and interspinal ligaments. Each segment was aligned with the intervertebral disc horizontal. Both segment extremities were potted with acrylic cement.

S1.2 -Surgical procedure
PCD is a surgery recommended for advanced degeneration of the disc, when the nucleus pulposus is replaced by a vacuum phenomenon [2,3]. As this specific state of degeneration is complicated to obtain in donor specimens, it was artificially created by manually emptying the disc to provide the anatomical vacuum characteristics needed for PCD using a substitutive method, thus providing a relevant and reproducible starting point. A rectangular incision as high as the disc and 5-8 mm wide was performed with a scalpel blade in the annulus fibrosus on the lateral side, preferably on the side showing irregularities (small osteophytes, wrinkled tissues). Lateral fenestration was chosen in consideration of the loading directions as it avoided damaging the tissue involved in flexion and extension biomechanics. The nucleus pulposus was extracted through the excision by a spine surgeon.
A highly radiopaque acrylic cement (Mendec Spine; Tecres, Sommacampagna, Italy, containing 30% BaSo4) was injected inside the disc through the incision until the cement would fill the cavity. The cement preparation was identical to clinical practice [3], mixing the components at room temperature, and waiting about 3-5 minutes to obtain the desired viscosity.
Supplementary Information to the paper: "Assessment of foraminal decompression following discoplasty using a combination of ex vivo testing and numerical tools" -3 - During each test, the 3-dimensional deformation distribution of the specimen surface were tracked using a Digital Image Correlation (DIC) system (Q400, Dantec Dynamics, Skovlunde, Denmark). For this purpose, a high-contrast speckle pattern was painted on both the vertebrae and the intervertebral disc using a methylene blue solution to stain the tissues and white waterbased acrylic paint sprayed on top [5,6]. Images were recorded at 15 Hz from the unloaded condition (reference frame) to the end of the 6 th cycle. The DIC-correlated image corresponding to the 6 th load peak was extracted from each test.

S1.3 -Biomechanical testing
Supplementary Information to the paper: "Assessment of foraminal decompression following discoplasty using a combination of ex vivo testing and numerical tools" -4 -

S1.4 -Post processing of DIC data
Fig. S1.1 -DIC data: from point clouds to identified vertebra surfaces. Supplementary Information to the paper: "Assessment of foraminal decompression following discoplasty using a combination of ex vivo testing and numerical tools"

S2.1 -Characterization of the DIC surfaces
As it was suspected that the quality of the overall registration could be affected by the quality of the DIC acquisition, the geometry of the correlated DIC surfaces was characterized in terms of dimension and unicity. The surface of the mesh was automatically measured by '3-matic'.
The total contour of the mesh was computed by adding the length of the external and internal 'bad contours' reported by the software. Finally, in order to assess the specificity of the surface, its "roughness" was measured. Roughness usually characterizes very small asperities however here, the evaluated geometric irregularities of the DIC-acquired surfaces were larger (of the order 1-5mm), and these features were important for the registration. By measuring the roughness at this level, the asperities were identified on the surface and quantified by their height. For that, the point clouds of the DIC surfaces were primarily segmented into 80 000 points in CloudCompare v2.6.0 opensource software (R&D Institute EDF, Paris, France, https://www.danielgm.net/cc/). The roughness, corresponding to the distance between the point and the best fitted plane on the kernel, was computed using 'tool>other>roughness' tool. In order to target the main asperities of the DIC surface, kernel sizes in the range of the asperities were tested. A 3.0 mm kernel size was finally set, allowing the identification of the asperities characterizing the vertebra shape while excluding the noise asperities created by the remaining soft tissue. Because both the height and the number of asperities helped the manual registration, the roughness distribution histogram was extracted for each surface, and the mean and maximum of the distribution were computed. In addition, to quantify the number of high asperities (called density of asperities below), the number of mesh nodes exhibiting a local roughness >0.5 mm was derived using a Matlab script.
The relationships between the HD values and the DIC mesh surface, HD values and the roughness parameters were investigated with Spearman's rank correlation.
Supplementary Information to the paper: "Assessment of foraminal decompression following discoplasty using a combination of ex vivo testing and numerical tools" -6 -

S2.2 -Correlation between DIC surfaces and registration precision
In order to have a more detailed assessment of the registration accuracy, the impact on the registration of the DIC mask characteristics were investigated. In particular, the total surface and the roughness of the masks were studied. The overall DIC mask surface area in the different specimens was at 440 ± 137 mm 2 (mean±s.d.). The maximum and mean roughness over all DIC masks were respectively 1.10 ± 0.36 mm and 0.11±0.02 mm. The mean density of asperities was 7973 nodes (range 0-64521). Weak but significant negative correlations ( Fig.   S2.1) were found between the DIC mask surface area and the mean HD values for O1 (ρ= -  regression is plotted for all data Supplementary Information to the paper: "Assessment of foraminal decompression following discoplasty using a combination of ex vivo testing and numerical tools" -7 -

S2.3 -Criteria on DIC surfaces
This analysis investigated the relationship between the DIC surface characteristics and the precision of registration. Among the studied parameters, only the area of the surface significantly impacted the registration repeatability. Unfortunately, this did not completely explain the outlier since 12 other specimens with lower surface area showed mean HD values in the overall range. Then, some other parameters should have probably also interfered and Supplementary Information to the paper: "Assessment of foraminal decompression following discoplasty using a combination of ex vivo testing and numerical tools" -8 -